Diluent Use in Thermal Recovery
Dual Use of Diluent—Treating and Transportation
Thermal recovery of heavy oil or bitumen is well known, for example by steam assisted gravity drainage (SAGD). Steam, provided into the subterranean reservoir, heats the heavy oil or bitumen, and the mobilized heavy oil or bitumen produced.
A diluent may be used to treat the produced heavy oil or bitumen to achieve a cleaner product, for example by treating or to form a blend for transportation, for example by pipeline or both for treating and transportation. Diluent comprising lighter alkanes is commonly used for said purposes. One such diluent is a light sweet synthetic crude, for example Suncor™ Oil Sands Blend A (OSA) or Syncrude™ Synthetic Crude (SYN). Another example of a diluent is natural gas condensate (NGC).
Diluent for Treating—Interaction with Produced Non-Condensable Gas (PG)
Typically, heavy oil and bitumen in a reservoir have some dissolved gas (primarily methane), which is evolved or liberated from solution during the process of oil recovery. Additional non-condensable gases may be generated within the reservoir, for example as a result of thermal recovery processes. At surface the presence of these gases results in vaporization of the lighter fractions of the diluent which is added to assist with emulsion separation. This gaseous phase is then separated from the liquids containing lighter fractions of diluent with the help of surface facilities such as air coolers and separators, as shown for example in FIG. 1.
Diluent for Transportation—Recovery from Blend Tanks
Typically, some portion of diluent from the tanks (vessels) provided for mixing the produced heavy oil or bitumen with diluent for meeting transport (e.g., pipeline) specifications flashes into vapors which is then compressed, cooled and separated into non-condensable gas and liquid hydrocarbons. Non-condensable gas then mixes with the other streams of non-condensable gas (PG) and is diverted to the fuel header, and the liquid hydrocarbons are diverted to the diluent tank, for example as shown in FIG. 1.
Solvent Use in Thermal Recovery
Various lighter hydrocarbon fractions have been used within a heavy oil or oil sands reservoir either to constitute the basis for a reservoir recovery process (i.e. used substantially alone) or to enhance the performance of an existing reservoir recovery process (e.g. used with, for example, steam).
Lighter hydrocarbons have been injected (without steam) with the aim of diluting and thereby mobilizing the heavy oil or bitumen, and ultimately recovering it using a gravity-dominated process, such as VAPEX™.
Lighter hydrocarbon fractions have also been used (with steam) to enhance the performance of steam-based reservoir recovery processes, including not only gravity-dominated processes, such as SAGD, but also those which are not gravity-dominated, such as Cyclic Steam Stimulation (CSS).
One reservoir recovery process known as Solvent Aided Process (SAP) involves use of lighter alkanes, such as propane or butane (indiscriminately chosen without specification of alkanes) with steam. Authors Gupta and Gittins have described a SAP process in a number of publications.
However, none of these solvent-based or solvent-aided processes provides concurrently both enhanced recovery (such as higher recovery efficiencies, lower SOR, or higher recovery factor) of heavy oil or bitumen from a reservoir and more efficient surface processing of the light hydrocarbon fraction that plays a role in achieving those enhanced levels of recovery from the reservoir.
In reservoir recovery processes where no solvents are injected, the non-condensable gas stream from the emulsion treating vessel, after cooling and separation, primarily contains only methane. However, when lighter solvents are used, this non-condensable gas stream is rich in those lighter solvents. Specifically, when lighter solvents such as C3 or C4 are used in conjunction with steam for enhancing performance of the oil recovery process from the reservoir, a large portion of these light solvents are mainly vaporized by the PG. Therefore in order to salvage the higher value in these light solvents, this stream needs to be processed in a ‘solvent recovery facility’ which separates the non-condensable gas from the liquid solvents, as shown for example in FIG. 2. The entire PG+solvent mixture is typically not used for re-injection into the reservoir, as the presence of the non-condensable gas compromises the miscibility of the solvent with oil in the reservoir.